The purpose of this study is to determine the efficacy of exercise training relative to pharmacologic treatment in clinically depressed older adults aged 50 years and older. This study is a randomized controlled trial of exercise training and/or pharmacologic treatment (sertraline or Zoloft) in which subjects undergo comprehensive assessments including a clinical diagnostic evaluation, exercise treadmill testing, and medical examinations. Following this assessment, subjects are classified as mild/moderately or severely depressed and within these groups randomly assigned to one of three treatment groups: (a) medication (Zoloft); (b) aerobic exercise training; or (c) medication and exercise. Following the completion of the 4-month treatment program, subjects undergo a third evaluation at 6-month follow-up. This study hypothesizes that: (1) an aerobic exercise program is feasible among older depressed patients, and the extent of improvement in cardiorespiratory function will be comparable to that of normal subjects; (2) an aerobic exercise program will be associated with comparable reductions in depression in mild-moderately depressed patients relative to mild-moderately depressed patients receiving medication alone; (3) drug therapy will be superior to exercise alone in the treatment of severely depressed patients; (4) aerobic exercise combined with medication will be superior to medication alone among severely depressed patients; (5) aerobic exercise training in mild- moderately depressed patients will be associated with less relapse at 6 months than patients receiving drug therapy alone; and (6) at 6 month follow-up, there will be greater relapse in the severely depressed patients receiving drug alone compared to patients who receive both drug and exercise training.